(a) Field of the Invention
The present invention relates generally to flat panel displays such as liquid crystal displays (LCDs), and more particularly, it relates to plane light source units for backlighting and a method for manufacturing holographic lightguides used for the same.
(b) Description of the Related Art
In recent years, flat panel displays have come into wide use as displays in electronic and electrical appliances, replacing conventional cathode ray tubes. Specifically, LCDs have been widely used in flat panel electronic displays.
LCDs comprise a plane light source unit as a backlighting source. Conventional plane light units for LCDs are shown in FIGS. 1A-1C. The conventional plane light source unit shown in FIG. 1A is adapted for an LCD having a relatively small or medium size, such as for a lap-top computer. Meanwhile, the plane light source unit shown in FIG. 1B or 1C is adapted for an LCD having a relatively large size, such as a monitor for desk-top computers, or a wide television monitor which can be attached to a wall.
First, referring to FIG. 1A, the conventional plane light source unit will be explained for an LCD having a relatively small or medium size. The conventional plane light source unit comprises a tubular light source 1, a light-guide plate 2 in which light from the tubular light source 1 propagates, and a reflecting plate 3 for reflecting light emitted from the tubular light source 1 to the LCD panel 8. The plane light source unit further comprises first and second diffusers 4, 5 and first and second prism plates 6, 7. Each prism plate has an array of microprisms formed on its upper surface. The edges of the microprisms are parallel to an x-axis in the first prism plate 6, while the edges of the microprisms are parallel to a y axis in the second prism plate 7 (see FIG. 2 (a)(b)).
FIG. 1B shows a conventional plane light source unit for an LCD having a relatively large size. The conventional plane light source unit comprises a pair of tubular light sources 1, 1xe2x80x2, a light-guide plate 2 positioned between the pair of the light sources 1, 1xe2x80x2, in which light from the light sources 1, 1xe2x80x2 propagates, and a reflecting plate 3 for reflecting light emitted from the pair of tubular light source 1, 1xe2x80x2 to the LCD panel 8. The unit further comprises two diffusers 4, 5.
Referring to FIG. 1C, another conventional plane light source unit for an LCD having a relatively large size is shown. The unit comprises a plurality of tubular light sources 1, a light-guide plate 2 above the light sources 1, and a reflecting plate 3 for reflecting light emitted from the plurality of tubular light sources 1 to the LCD panel 8. The unit further comprises two or more diffusers 4.
In the case of a conventional diffusion type light-guide plate used in the conventional plane light source units, scatterer patterns are formed on the reflecting surface on the light-guide plate in order to scatter light for uniform illumination. The scatterers are illustrated in FIG. 3. The light-guide plate may have depressions like (a) prism type or (b) semi-sphere type. The light-guide plate may have an ink dot pattern in which ink including scattering material is printed ((c) ink printing type).
The operation of the conventional plane light source unit is as follows:
First referring to FIG. 4, the plane light source unit as shown in FIG. 1A will be described. The light emitted from the tubular light source 1 enters the light-guide plate 2 and reflects internally (ray R1) in the light-guide plate 2. The light exits to the reflecting plate 3 when the light does not satisfy the total reflection condition or impinges on the scatterer of the light-guide plate 2 as shown in FIG. 3. The light is then reflected on the reflecting plate 3 and may reenter the light-guide plate 2 (ray R2). On the other hand, when the light reflected internally in the light-guide plate 2 impinges on the upper surface of the light-guide plate 2 and does not satisfy the total reflection condition, the light exits from the light-guide plate 2 (ray R3) with an angle xcex81. The light then enter a first diffuser 4 and scatters with an angle xcex82 (ray R4) which is larger than the angle xcex81. The light through the first diffuser 4 enters a first prism plate 6. Since the first prism plate 6 has edges of the microprisms parallel to the x-axis, rays in the y-z plane are affected to decrease the scattering angle, but rays in the x-y plane are not affected (ray R5). The light though the first prism plate 6 enters a second prism plate 7 to make the main ray parallel to the y axis (ray R6) since the second prism plate 7 has edges of the microprisms parallel to the y-axis. Then the light is finally scattered by the second diffuser 5 to have a uniform light distribution for LCDs.
Referring to FIG. 5, the plane light source unit as shown in FIG. 1B will be described. The light emitted from the two tubular light sources 1, 1xe2x80x2 enters the light-guide plate 2 and reflects internally in the light-guide plate 2. The light exits to the reflecting plate 3 when the light does not satisfy the total reflection condition or impinges on the scatterer of the light-guide plate 2 as shown in FIG. 3. The light proceeding to the reflecting plate 3 is reflected from the reflecting plate 13, and may then re-enter the light-guide plate 2 (ray R2). On the other hand, when the light reflected internally in the light-guide plate 2 impinges on the upper surface of the light-guide plate 2 and does not satisfy the total reflection condition, the light exits from the light-guide plate 2. The light intensity distribution at the upper surface of the light-guide plate 2 is shown in (b) of the FIG. 5. The lights then enter a first diffuser 4 and scatter to have the light intensity distribution shown in (c) of FIG. 5, which is more uniform than (b). The light that passes through the first diffuser 4 and enters a second diffuser 5 has a substantially uniform light intensity distribution as shown in (d) of FIG. 5, resulting in uniform illumination for LCDs.
As described above, since a light-guide plate of the conventional plane light source unit has scatterers having depressions like prisms or semi-spheres, it is required to have time-consuming and high cost manufacturing processes. Conventional injection molding methods or shaping methods with diamond cutters are not suitable for making light-guide plates which have fine depressions such as on the order of magnitude 10 xcexcm.
On the other hand, it is also a time-consuming process to print dot patterns in the case of the ink printing type light-guide plate. Further, the ink printing type light-guide plate has drawbacks of overall low efficiency since both the ink and the scattering material may absorb light.
Now referring to FIG. 6, the plane light source unit as shown in FIG. 1C will be described. The light emitted from the tubular light sources 1 enters directly through the light-guide plate 2 (ray R2), or reflects from the reflecting plate 3 to the light-guide plate 2 (ray R1). When the light passes through the light-guide plate 3, the light distribution intensity is high near the light sources 1 as shown in diagram (C). The light passes two or more diffusers and then has a substantially uniform light distribution as shown in diagram (b), resulting in uniform illumination for LCDs.
However, since light is absorbed by the scattering pattern, the utilization efficiency of light is poor, and the power consumption of the light source is large.
Further, since the conventional plane light source units need several diffusers or prism plates to obtain uniform illumination, the light is absorbed by the diffusers or prism plates, and this results in a degradation of the brightness of the LCDs.
Therefore, it is required to use a higher intensity light source or more light sources, resulting in cost increases, and increased power consumption.
Further, since diffusers or prism plates are generally expensive, it increases the entire manufacturing cost.
In view of the prior arts described above, it is an object of the present invention to provide an improved plane light source unit which provides more energy efficiency and uniform illumination.
It is another object of the present invention to provide an improved plane light source unit having high brightness.
It is further another object of the present invention to provide a holographic light-guide used in the improved plane light source unit and method thereof.
It is still another object of the present invention to provide a holographic diffuser used in the improved plane light source unit and method thereof.
To achieve these objects, as embodied and broadly described herein, the invention comprises
at least one tubular light source, elongated in a direction;
a holographic light-guide having at least one light incident surface placed near the tubular light source, a reflecting surface on which a first holographic layer is formed for uniform illumination, and a light emitting surface;
a reflector disposed against the reflecting surface of the holographic light-guide, for reflecting the light emerged from the reflecting surface and redirecting the light to the reflecting surface,
wherein the first holographic layer has patterns formed with a plurality of holograms, the area density of the pattern being low toward the tubular light source and increasing with an increase in distance from the tubular light source.
According to another aspect of the present invention, a plane light source unit comprises
a plurality of tubular light sources disposed in parallel, elongated in a direction;
a holographic light-guide having a light incident surface placed against the tubular light sources and a light emitting surface opposite to the light incident surface, a first holographic layer being formed on either the light incident surface or the light emitting surface; and
a reflector disposed against both the tubular light sources and the reflecting surface of the holographic light-guide, for reflecting and redirecting the light to the reflecting surface,
wherein the first holographic layer has speckles, the speckles being formed densely near the tubular light sources and sparesely farther from the tubular light source.
According to further aspect of the present invention, a holographic lightguide for plane light source units comprises the steps of:
1) forming a first hologram for the first holographic layer, comprising the steps of:
A) enlarging a laser beam and entering it into a diffuser;
B) exposing a first substrate with a mask having patterns to be formed to the laser beam, the first substrate being coated with photoresist;
C) developing the first substrate by etching the photoresist;
2) forming a second hologram for the second holographic layer, comprising the steps of:
a) enlarging a laser beam and sending it into a cylindrical lens so that the laser beam enters a diffuser;
b) exposing a portion of a second substrate with a shielding plate to the laser beam at a predetermined distance, the second substrate being coated with photoresist;
c) repeating the step b) as the distance increases by a predetermined increment until the entire surface of the second substrate is exposed, wherein the portion to be exposed is adjacent to the portion previously exposed and the portion previously exposed is shielded by the shielding plate;
d) developing the second substrate by etching the photoresist;
3) forming a first metal stamper by electroless-plating the first substrate;
4) forming a second metal stamper by electroless-plating the second substrate;
5) coating a transparent plate with an ultraviolet-curing material;
6) irradiating the coated transparent plate by ultraviolet rays, but not to the extent that the ultraviolet-curing material is completely cured;
7) pressing the irradiated transparent plate up and down by the first metal stamper and the second metal stamper simultaneously; and
8) irradiating the transparent plate by ultraviolet rays until the ultraviolet-curing material is completely cured.
Both the foregoing general description and the following Detailed Description are exemplary and are intended to provide further explanation of the invention as claimed.